This invention relates to the use of a chirped synthetic aperture radar (SAR) to measure ocean current velocity from spacecraft, and more particularly to a SAR system for measurement of surface currents of the ocean from the ocean wavelength of 2.pi./.DELTA.k, where .DELTA.k is the difference in wavenumber between two frequencies f.sub.1 and f.sub.2 of a chirped SAR.
Surface currents of the ocean vary widely from the current velocity below about one meter due to local surface wind and wave fields that differ from the normal waves resulting from geostrophic forces and tides. It would be desirable to map the deeper ocean current velocity from a spacecraft not only for climate prediction in coastal areas, but also for navigation and management of fishing operations.
Previous work has shown that .DELTA.k radar techniques can measure current velocities, and determine dominant wavelengths, based on the fact that if the Bragg angle .theta. between the incoming radiation and the mean surface is larger than the rms surface slope, but smaller than about 70.degree., microwave scattering is well defined by the Bragg law, as discussed by W. J. Plant and D. L. Schuler in a paper titled "Remote sensing of the sea surface using one- and two-frequency microwave techniques" published in Radio Science 15, 605 (1980).
In that paper, the authors describe a dual-frequency technique. Two coherent microwave signals are transmitted from a stationary CW radar, and signals scattered from the same spot at the two frequencies are multiplied to produce a spectrum with a sharp line, the .DELTA.k line. The authors show that for a narrow beam and relatively long observation times, a dual-frequency system may thus be used to measure current velocities and determine dominant wavelengths.